Frequency, or phase, deviation changer circuit



Dec. 14, 1943. M. G. CROSBY FREQUENCY, OR PHASE, DEVIATION CHANGER CIRCUIT Filed Feb. 11 1942 2 Sheets-Sheet 1 Y m R Y ma, m R. m /h m\\\ m I QR @QQ w Dec. 14, 1943. M. G. CROSBY FREQUENCY, on PHASE, DEVIATION CHANGER cmcum 2 Sheets-Sheet 2 Filed Feb. 11, 1942 QEQEEQQQQL lNVENTOR x. H H

ATTORNEY wE F Patented Dec. 14, 1943 anger OFFICE FREQUENCY, 0R PHASE, DEVIATION CHANGER CIRCUIT Murray G. Crosby, Riverboat],

N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application February 11, 1942, Serial No. 430,348

18 Claims.

My present invention relates generally to circuits for changing the frequency, or phase, deviation of frequency modulated, or phase modulated, carrier waves respectively, and more especially the invention relates to deviation changer circuits of the type wherein a pair of frequency converters are arranged in an oscillating system wherein an incoming modulated carrier wave is utilized as one of the sources of heterodyning oscillations.

In the prior art of frequency and phase modulation the process of frequency multiplication has been used to increase the degree of modulation, while the process of frequency division has been employed to decrease the degree of modulation of the carrier wave. It is, also, known to reduce frequency, or phase, deviation by a lockedin oscillator method. In this method an oscillator at a sub-harmonic frequency is held in step with the applied modulated carrier waves; automatic frequency control may be applied to the oscillator to aid the oscillator in following the applied modulated carrier waves. In such a system a beat note may be effected at the instant the applied modulated Waves lose control of the local oscillator, the beat note being effected between the applied waves and the oscillatcr frequencies which may be slightly different. My present system eliminates the possibility of such beat note being produced, since the local oscillations are not produced at the instant the applied signals lose control.

It is one of the main objects of my present invention to provide a frequency, or phase, deviation changing circuit wherein a pair of frequency converters are arranged-in an oscillating system, and the applied modulated waves act as the source of local oscillations of one of the converters; the converters being arranged to be reentrant so that oscillations are sustained.-

Another important objectof my invention is to provide a pair of. frequency converters in a 're-entra'nt oscillation system; the system funotioning to remove amplitude modulation from applied angular velocity-modulated carrier waves without the aid of a limiter network per se.

Another important object of the invention is to provide a system for reducing the frequency deviation of a frequency modulated carrier wave, the deviation reducing system comprising oscillation network under the control of the applied modulated waves, and wherein sustained oscillations of the oscillation network cease when the applied modulated waves lose control thereby permitting adjustment such that-only applied waves above a predetermined intensity may be transmitted.

Still another object of the invention is to provide a network for reducing the frequency deviation of applied angular velocity-modulated carrier waves; there being utilized at least two frequency converters arranged in a re-entrant circuit so that oscillations are sustained, and the applied modulated waves act asthe source of local oscillations for one of the converters thereby enabling inter-station noises to be eliminated by virtue of adjustment such that only applied modulated carrier waves above a predetermined noise level may be received.

Still other objects of the invention are to if prove generally the emoiency and reliability of frequency, or phase, deviation changer circuits, and more especially to provide receivers of angular velocity-modulated carrier waves which are economically manufactured and assembled.

The novel features which I believe to be characteristic of my invention are set forth in par ticularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawings in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into eifect;

In the drawings:

Fig. 1 shows a receiving system embodying one form of the invention,

Fig. 2 illustrates a modification of the deviation changer circuit.

Referring, now, to the accompanying figures, wherein like reference characters in the two figtires-designate similar circuit elements, Fig. 1 shows a receiver system of the superheterodyne type. Ingeneral, it may embody a signal collector device I, which may be a di -pole, a grounded antenna circuit, or even an ultra-high'r'adio frequency distribution line. v The collected signals, which may be frequency modulated, or phase modulated, carrier waves, are applied to the radio frequency amplifier 2. Amplifier '2 may embody one or more'stages of tunableradio frequency amplification. The generic term angular velocity-modulated carrier wave is to be understood as denoting carrier waves which have been'frequency modulated or phase modulated. Those skilled in the art are fully aware of the fact that a frequency modulated carrier wave (FM) has its carrier, or mid-band, frequency Fe instantaneously varied, or deviated, in .fre-

, The rate of frequency carrier, or mid-band, frequency limiter stage. This was 2.. quency value in accordance with the amplitude of the modulating voltage at the transmitter. deviation depends upon In phase deviated to the modulation frequencies per 'se. modulation (PM) the frequency is a greater extent at the quencies; otherwise, this form of, modulation is similar to frequency modulation. It'willlbe understood that the term"-deviation refers to the maximum applied frequency deviation in the case of a frequency modulated wave, and to the higher modulation fremaximum applied phase deviation in the casc of a phase modulated wave.

In so far asthe channel width of each station is concerned, it will be understood that the frequency deviation may be high or low. Thepresent practice in FM broadcast transmission is to employ carrier frequencies in the range of 42-50 megacycles (mc.) The carrier frequency is deviated a maximum of '75 kilocycles (kc.) to either.

side, and each channel width is 200;kc. ,1'Ihatis, the maximum overall frequency swing- ,is 150 kc., whereas the allotted channel width is 200 kc. )It is .to be clearly. understood that these are purely illustrative values, and can upon the circumstances which face the receiver designer. Assuming, however, this application that the frequency deviation is '75 kc., and that the carrierv is chosen from the 42-50 mc. band, the amplified modulated carrier waves are appliedto a converter 3 to reduce the of the modulated waves tothe'operating intermediate frequency (1. F.) value. Those skilled in the art are fully aware of the manner 'of constructingthe converter stage, and it. is .to be understood that it may utilize a separate local'oscillator and a separate first detector, or the converter may be a combined local oscillatorefirst detecter stage which uses the wellknown pentagrid converter tube.-

By way of illustration the I. F. value is chosen as 13 me. The I. F. amplifier 4 may comprise one or more stages of amplification. The numeral 5 denotes the I. F, output transformer whose primary and secondary circuits are each tuned to 13.mc., which is the carrierfrequency of the modulated waves. Adjacent to theqtransformer 5 thereis shown in a purely pictorial man-' ner the appearanceof theresponse characteristic of transformer 5. former, as well asits priorcircuits, produces a flat-topped band pass characteristic having an be varied depending for the purpose of It will be seen that'the translatter comprises a coil I6 shunted by denser I'I."

seen that tubes I and 2 are each pentagrid tubes. Tube I' has its third grid 6 connected to the high potential side of the secondary circuit of I. F.

transformer 5, thelow potential side of the secondary circuit being grounded. The first grid I is connected by grid condenser 8 to an intermediate tap 9 on the coil II] of resonant circuit 4. The condenser II in shunt with coil I resonates circuit 4 to a frequency of 1'7 mc. The

'grid leak resistor I2 connects the grid to the I grounded end of biasing resistor I3, the latter 7 being Icy-passed by condenser I4 for I. F. currents.

The, plate I5 of tube I is connected to the high potential side of resonant output circuit 3. The

tuning con- The low potential side of tuned circuit 3 is con- 1 nected to the positive terminal of the direct curover-all band width of 150kc. with Fe at 1 3 mc. I

The frequency deviation is representedas being In the prior art it has been customary to apply the output of the, I. F. amplifier to an amplitude often constructed in the manner of a readily .saturatable amplifier,

which has an input-output characteristic whichis fiat above a predetermined wave input amplituda; According to my invention, however such a special limiter ,stage is eliminated- Prior to ;the usual discriminatorrectifier network thereis employeda network for reducingthe deviation band width'of the energyoutput of .transformeri. Concurrently,"this deviation reducing network effects a limiting action.

Referring to the. diagram in Fig. 1,. it will be,

done inorder to remove "the grids 6 and 22 rentenergizing source, the condenser I8 by-passing the low potential side of circuit 3 to ground for radio frequencycurrents. The circuit 3 is resonated to a frequency of 4 mc., and the circuit is. given avpass. band of,28.6 kc. The response characteristic of circuit 3 is depicted adjacent to it. The plate I9 of tube 2 is connected to the high potential side of resonant circuit 4', while the low potential side thereof is connected to the positive terminal of a direct current energizing source. It will be understood that a common direct current energizingsourcemay be used for the cold electrodes of converter tubes I' and 2. The condenser may by-pass the energizing connection to plate I9 for radio frequency currents, and biasing resistor I3, shunted by I. F. by-pass condenser I4, may-functionto provide the grid bias for the spaced control grids 2| and 22 of tube 2.

The third grid 22 is connected to the high potential side of the secondary circuit of I. F; transformer 5. .The directcurrentvoltage developed across" resistor I3 provides the normal grid bias for grid 22. The grid 2I, which is the first grid of converter tube 2, is connected to an intermediate tap 24 on coil I6 of circuit 3'. Grid condenser 25 is inserted in the connection of tap 24.

I, which is developed across resonant circuit 3',

is fed to'the input electrodeZI of converter tube 2'. Furthermore, the output voltage of converter tube 2', which is developed at resonant circuit I3, is applied to the grid! of converter tube I. The applied modulated carrier Waves are fed to of both converter tubes, and is the I. F. energy developed by the I. F. transformer 5. Thisarrangement is proper for the production of a heterodyne' output-between the applied signal waves and' the oscillations on the spaced control grids of each of the converter tubes.

With the resonating of tuned circuits 3' and 4' as describedthe system including. tubes I and 2 is re-entrant so that oscillations will be sustained; The conversion gain of the converter tubes is assumed to be greater than unity. Hence, a pulse. of voltage withinthe circuits causes a component to appear, for instance atl'l mo, in

tions; It'usually is supplied by the surge of current which takes place when the: oscillator is turned on. The 17' mo. voltage across'd hetero dyneswith the 13mm signal voltage in'tube I fed to grid 6, thereby producing a 4 mo. componnent in tuned circuit 3. The 4 mc. component heterodynes with the 13 mc. signal voltage in tube 2', fed to grid 22, to produce an amplified -17 mc. component across tuned circuit '4. Inthis way oscillations are built up until some part of the re-entrant circuit limits the amplitude at its sus tained oscillation value. The result is asystem which is in oscillation at the two frequencies of tuned circuits 3 and 4', i. e., 4 mc. and l'l'mc. respectively, for the case of the example assumed here.

The existence of these oscillations is dependent upon the presencef'of the signal input voltage,

since without the latter there would beno heterodyning action. It is, also, dependent upon the frequency of the signal voltage, since a shift in frequency of-the latter will require that the tuned circuits be operated on the side of curves instead of at resonance. However, oscillations will be maintained as long as the frequency is such that the resultant conversion gains effect enough amplification to make the re-entrant effect cumulative. The point of loss of the cumulative effect will occur when the circuits have become so far detuned that the combination of the gain and phase conditions causes each re-entrant circulation to be less than the preceding one. Thus, it will be apparent that the signal input frequency may vary over a range which is appreciable, as long as it does not cause the tuned circuits to be detuned too far.

As the signal input frequency is varied, the frequency of the oscillations in both tuned circuits 3 and 4' will also vary. The amount that each Varies will depend upon the selectivity of its tuned circuit. In order clearly to point out the functioning of the re-entrant system a mechanical analogy is herein presented. The analogy is purely illustrative. The system is analogous to the drive of two flywheels of of a differential drive gear. sume that the rear axle of jacked up,

For instance, asan automobile is and that one rear wheel is replaced by a heavy flywheel and the other by a light flywheel. At a given motor speed both flywheels will turn, but the lighter one will turn the fastest. If the motor speed is increased, the lighter flywheel will take most of the increase. The heavier flywheel would be considered the most stable of the two, and would have the least speed variation. Now, in the case of the oscillating converters l and 2, the low-frequency tuned circuit 3 will normally have the highest selectivity which is analogous to the heavy flywheel different size by means" the selectivity efiect. Consequently, when the frequency of the input signal is changed, the larger part of the change will take place on the higher-frequency tuned circuit 4 since its flywheel effect is less.

Another way of describing the operation is as follows: If both tuned circuits had equal values of Q the low-frequency circuit ,3 would have the highest degree of selectivity, since a given frequency change in cycles would represent a larger percentage frequency change in-that circuit. When the input signal frequency is changed,- both circuit frequencies will change. However, since the oscillation follows the path of least resistance and assumes the frequency which is a compromise between the maximum possible gain and the best phase position, the lower frequency will shift to approximately the same amplitude and phase on its tuned circuit that the higher frequency does. For this to be variations.

the case the lower frequency cannot shift as far in frequency, since it is relatively more selective.

In the numerical example, assume that the 13 mc. signal frequency shifts 75 kc. higher. If the selectivities of the tuned circuits 4 and 3' were the same (by having values of Q proportional to their frequency) the 4 mc. voltage would shift to 4 mo. minus 37.5 kc. and the 17 mc. voltage would shift to 17 me. plus 37.5 kc. This would be the equilibrium condition. However, if the values of Q'of the circuits were equal, so that the 4 mo. circuit was times as selective as the circuit 4, then the two frequencies would shift in the ratio of 4 to 17. This would make the 4 mc. circuit shift approximately 14.3 kc., and the higher frequency would shift approximately 60.7-kc.

From the above it can be seen that the frequency deviation of the input signal has been reduced in the case of both of the resulting oscillations. The lower frequency oscillations have been reduced approximately 4:1, whereas the carrier frequency would be reduced in the ratio of 17:4 or 4.25 to 1. Thus, a reduction of frequency deviation is effected which is even greater than the reduction of the carrier frequency. In the case of the usual type of frequency multiplication or division, the frequency deviation change is the same as the order of the frequency multiplication or division. According to the present method of reducing the degree of deviation, however, such reduction can be carried to any desired degree by adjusting the relative selectivities of circuits 4' and 3'. Such an adjustment may be effected by varying the damping of the two tuned circuits. Thus, if it were desired to decrease the deviation of the circuit 3", the damping of the circuit A could be increased by lowering the shunt resistance across circuit 4', or by increasing the series'resistance in circuit 4'.

In experimental work with this circuit, it has been observed that the direction of the frequency deviation is opposite on the two oscillation frequencies. This furnishes a means of reversing the direction of a frequency, or phase, modulation deviation. Experimental work has, also, shown that once the oscillations have started in the reentrant system, the amplitude of oscillations is quite insensitive to amplitude variations .of the modulated carrier wave input. As the wave input is brought up oscillations start and increase in amplitude for a small range, but limit to a constant value. Thus, the system has the property of passing only the frequency, or phase, variations of the input waves thereby eliminating amplitude Hence, there is shown above tube I a characteristic which shows a typical limiting characteristic relating input and output wave amplitude.

In the receiving system the voltage developed across tuned circuit 3, which would have the carrier frequency of 4 mo. and a maximum frequency deviation of 14.3 kc. instead of 75 kc., would be fed to a succeeding demodulation network. Such a demodulation network could be of any type well known to those skilled in the art. Usually it comprises a discriminator-rectifier, and the discriminator would have its input circuit tuned to a frequency of 4 mc., and would be adjusted to handle a maximum deviation of 14.3 kc. It is not Joelieved necessary to show the specific details of any discriminator network, since the various types of demodulator networks are well known to those skilled in the art. For example, there may be utilized the type of discriminator-rectifier would also have been 211110., if

4- shown by S. 3 W. Seeley hisgU. S. Patent; 2,121,103, granted June 21,1935; ;It will hemmed; that the demodulatoris not responsive to thegfull frequency variations of the wave as 'collectedby: the signal collector. LFllIthGI', no limiter per se need be used prior to the demodulator. Addi tionallyinter-channel noise squelchingjssfiilllred;

It will be apparent that this same type ofdevia tion reducing circuit may ceiver. The use of. such a deviationreducing system in a PM receiver has the advantage that higher values of phase deviation maythen be used without overloading the phase detector. The'function of the deviation reducing network in suchcase,' and also in the; case of an FM re-- ceiver, is the same as that 'of the frequency divider disclosed in my U. S. Patent 2,230,231. For the case ,ofphase modulation reception, the use of sucha frequency division increases the phase deviation capabilities f a, phasemodula tion receiver which is normallylirnited to a maxi e a l t a PM;re-

mum phase deviation of one radian. Receivers of the crystal filtertypaand others, have this limitation. Hence, by the inclusion of frequency divisionhig h-deviation, phase modulation is-made possible on these types of receivers,

Where it is desired to obtain extreme reduction of deviation, the circuit of Fig.2 may be employed. In this tube 5 is provided which produces a heterodyne output which is the sum of theltwo oscillation frequencies. That is, the output of converter S will be the sum of the oscillations produced-across circuits 4 and 3. Therefore, theheterodyneoutputwould be equal to 21fmc., andwould-be developed across tuned circuitffi' arrangedin the plate circuit of converter tube 51.x The, output of circuit 6' willbe constant in frequency if the selectivities of the oscillation 'circuits {and ,4 are the same. 'I'hereason for this is-thatthe deviation of the output of circuit 6' is the difarrangement} a third converter I ing from the scope of my invention, as set forth ference of the deviation present inthe oscillation circuits 3 and 4". r V v For instance, in the case of the previouslymentioned example for equal yalues of selectivity in circuits 3 and 4", the 13 mo. input'deviated 75 kc. higher, the, lowerfrequency deviated 37.5 kc. lower, and the higher frequency deviation 37.5 kc. higher. The sum of 4 me. minus 37.5 kc. and 17 mc. plus 37.5 kc. is 21 mo. The sum frequency the 13 me. signal had not deviated at all. For the otherexample, in which thelower frequencydeviated 14.3,kc. and the higher frequency deviated 60.7 kc. higher, the sumof the two oscillation frequencieswould be 21 me. plus 46.9 kc, For this case a deviation of 75 kc. inthe signal frequency produced a i611 kc. deviation in the output frequency. 1 a; w i Hence, in the system of Fig. 2 as the selectiv ities of the two oscillation circuits 3' and i; are made more unequal,the suin'frequency; output developed across circuit a larger deviation,- of the selectivity, of thetwo osciilationcircuits-3 and 4, the deviation maybe controlledfrom a value of zero up to approximately the'san e devia tion as the signal. It will therefore. be seen that the'nearer to equality that the :two selectivities are, the greater is the reduction ofthedeviation. In the circuit of Fig. 1 the opposite is true. That, is, the greater the difference the selectiv ities; the greater the deviation reduction.

la c y r i ha Consequently by adjustment.

One .ofthese is for a tone .ing a original frequency,

has its first grid '30 coupled through condenser 3l ;to tap 24 ofcoil Hi. The third controljgrid 32 isconnected by condenser 33' to the tap; Sgon 0011510 of circuit 4. Grids 32 and 3B are,returned by resistors'to the grounded end of the biasing resistor arranged in the cathode circuit of converter tube 5'. Ofcourse, heterodyne output Voltage developed across resonant circuit 6} is transmitted toany well known type of FM g iscrirninatonas explained in connection with In both Figs. 1 and 2 the taps 24 and 9 serve to reduce the overall gain of the cascaded converters. They, also, ingthe grid circuits, which the taps feed,- from loading the tuned circuits so as to reduce their selectivity. The overall gain is usually much higher than is required for sustained oscillations so that. tapping down,:with its advantage of improved selectivity, may be employed. l

There are other uses to which the re-entrant systemshown in this application is applicable. keyer for telegraph reception. The threshold effect and the selflimiting feature are both ideally suited for this operation; In this usage the keyed signal is fed to the signal input, and the frequency variations are removed'by the deviation-reducing effect of the re-entrant system.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my inventionis by no means limited to the particular organizations shown and described, but that many modifications may be made without departverter' tube ,5

in the appended claims.

What I' claim is:

1. In a system of frequency, or phase, deviation reductiomthe'method which include heterodyn- I or phase, modulated car-. rier waves of a given mid-band frequency and frequencyideviation with oscillations of a higher frequency than said mid-band frequency, deriving'from the heterodyne energy modulated waves whose mid-band frequency is equal to the differ ence of said given frequency and higher frequencyv and whose deviation is substantially less than said given deviation, and heterodyning said derived modulated waves with said original waves thereby to'produce said oscillations of higher frequency. Z g In'a system of frequency, or phase, deviationreduction, the method which includes heterodyning original frequen cy or phase, modulated carrier'waves of a given carrier frequency and deviation with oscillations of a higher frequency than said carrier frequency, deriving from the heterodyne energy modulated waves Whose carrier frequency is equal to thedifference of said given frequency and highe frequency and whose deviation is substantially less than said given deviation, heterodyning said-derived modulated waves with said original Waves thereby to produce said oscillations of higher frequency, and finally combining said higher frequency oscillations with said derived waves to produc a second hetero-' dyne voltage which is the sum of the combined frequencies.

3. In a system tion reduction, the method which includes hetero-- dyning original frequency,'or, phase, modulated Considering the systemshown in Fig. 2 it will be noted that the circuits of converter tubesl carrier waves of a given frequency and frequency swing with oscillations of a higher frequency than said given frequency, deriving from the hetero- Ia dyne energy if modulated waves whose frequency,

have theeffect' of prevent of frequency, or phase, deviaassesses is equal to the difference of said given frequency and higher frequency and whose frequency swing is substantially less than said given frequency swing, heterodyning said derived modulated waves with said original waves thereby to produce said oscillations of higher frequency, and maintaining said derived waves and said higher frequency oscillations at different degrees of damping.

4. A method of reducing the frequency deviation of original angular velocity-modulated carrier waves of a pre-determined frequency and a given frequency deviation, which includes deriving from said modulated Waves similar modulated carrier waves whose frequency is higher than said predetermined frequency but whose frequency deviation is lower than said given deviation, combining said original waves with the derived waves thereby to produce resultant modulated waves at a reduced frequency equal to the difference of said predetermined and higher frequencies and whose frequency'deviation is reduced from said given deviation, and deriving from the combination of said original waves and said resultant waves said higher frequency modulated waves.

5. A method of reducing the frequency deviation of angular velocity-modulated carrier waves of a predetermined carrier frequency and a given carrier frequency deviation, which includes deriving from said modulated waves similar modulated carrier Waves whose carrier frequency is higher than said predetermined frequency but whose frequency deviation is lower than said given deviation, combining said original waves with the derived waves thereby to produce resultant modulated waves at a reduced carrier frequency equal to the difference of said predetermined and higher frequencies and whose deviation is reduced from said given deviation to a greater extent than said reduced carrier frequency, and deriving from the combination of said original waves and said resultant Waves said higher frequency modulated Waves.

6. In combination with a pair of electron discharge tubes each having a pair of input electrodes and an output electrode, means applying signals having a given frequency variation to an input electrode of each tube, a separate resonant circuit connected to each output electrode of each tube, said resonant circuits being tuned to respectively higher and lower frequencies than the mean frequency of said applied signals, a coupling connection from each resonant circuit to the second input electrode of the opposite tube, and means for deriving from the lower frequency-tuned circuit signals whose frequency variations have been reduced relative to said given frequency variation.

7. In combination with a pair of electron discharge tubes each having a pair of spaced control electrodes and an output electrode, means applying signals of a predetermined frequency variation to a control electrode of each tube, a separate resonant circuit connected to each output electrode of each tube, said resonant circuits being tuned to respectively higher and lower frequencies than the mean frequency of said applied signals, a connection from each resonant circuit to the second control electrode of the opposite tube, and means for deriving from the lower frequency resonant circuit signals whose frequency variations have been reduced relative to said predetermined frequency variation, and means for adjusting the relative selectivities of said two resonant circuits.

8. In a system of frequency, or phase, deviation reduction, the method which includes het-' erodyning original frequency, or phase, modulated carrier wave of a given mid-band frequency and frequency deviation with oscillations of a higher frequency than said mid-band frequency, deriving from the heterodyne energy modulated waves whose mid-band frequency is equal to the difierence of said given. frequency and higher frequency and whose deviation is substantially less than saidgiven deviation, heterodyning said derived modulated waves with said original waves thereby to produce said oscillations of higher frequency, and providing predetermined unequal damping of said derived waves and last-mentioned oscillations thereby to reduce said given deviation to a predetermined lesser value.

9. A method of reducing the frequency deviation of angular-velocity modulated carrier Waves of a predetermined carrier frequency and a given carrier frequency deviation, which includes deriving from said modulated waves similar modulated carrier waves whose carrier frequency is higher than said predetermined frequency but whose frequency deviation is lower than said given deviation, combining said original waves with the derived waves thereby to produce resultant modulated waves at a reduced carrier frequency equal to the difference of said prede-' termined and higher frequencies and whose deviation is reduced from said given deviation to a greater extent than said reduced carrier frequency, deriving from the combination of said original waves and said resultant waves said higher frequency modulated Waves, and maintaining said resultant waves and higher frequency Waves at predetermined different dampings.

10. In combination with a pair of electron discharge tubes each having a pair of spaced control electrodes and an output electrode, means applying frequency modulated carrier signals of a given frequency deviation to a control electrode of each tube, a separate resonant circuit connected to each output electrode of each tube, said resonant circuits being tuned to respectively higher and lower frequencies than the mean frequency of said applied signals, a connection from each resonant circuit to the second control electrode of the opposite tube, means for deriving from the lower frequency resonant circuit signals whose frequency deviation has been greatly reduced relative to the given deviation, means for adjusting the relative selectivities of said two resonant circuits, thereby to determine the deree of deviation reduction.

11. In a system of frequency, or phase, deviation reduction, the method which includes heterodyning original frequency, or phase, modulated carrier waves of a given center frequency and frequency deviation with oscillations of a higher frequency than said center frequency, deriving from the heterodyne energy modulated waves Whose center frequency is equal to the difference of said given frequency and higher frequency and whose deviation is substantially less than said given deviation, heterodyning said derived modulated waves with said original waves thereby to produce said oscillations of higher frequency, and combining said last oscillations with said derived modulated Waves to produce modulated waves of greatly reduced deviation relative to said given frequency deviation.

12. In a system of frequency, or phase, deviation reduction, the method which includes mixing original frequency, or phase, modulated carrier Waves of a given center frequency and deviation applying signals input electrode with'oscillationsio'f ahigher frequency than said center frequency, deriving from'the mixed energyymodulated waves. whose center [frequency is equal to the difference iof'said given frequency and higher f frequency and whosedeviation is substantiallyxless than said given deviation, mixing charge ,tubes eachhaving a pair of input elecmeans applying trodes and, an output electrode, frequency 'modulated carrier waves to'an input electrode of each tube, separate resonant circuits, connected to each output electrode of each tube, tuned to-irespectively higher and lower frequencies than, the carrier frequency of said applied waves, a coupling resonant circuititothe second input electrode of the opposite tuba-and means for mixing the waves produced across each ofthe separate resonant circuits. to produce resultant frequency modulated waves whose frequency deviationis greatly reduced relativeto the frequency, deviation of said applied waves; 7 i

14. A re-entrant circuit comprising a pair of electron discharge tubes each having a pairof input electrodes and an output electrode, means having a given frequency deviation to an input electrode of each tube, a separate output circuit connectedto each output electrode of each-tube, said output circuits being tuned to respectivelydifferentfrequencies than the mean frequency of said applied signals, a coupling connection fromeach output circuit vto the second of the opposite tube, and means for deriving from at least one of the output circuits signals whose frequency deviations have been reduced relative to said given frequency deviation.

15. A re-entrant circuit comprising a pair of converters, each converter having an input. circuit and anoutputcircuit, means feeding frequency modulated carrier signals to be heterodyned to the input circuit of each converter, said i signals having a predetermined frequency deviation, means feeding the heterodyne output'of the output circuit of one converter to the input circuit of the second converter, means feeding connection from each 'quency deviations which differ from said firstnamed frequency deviation, and deriving each of the last named waves from the heterodyne energy of the opposite heterodyne step.

17. In combination with" a pair of tubesea'ch having; a pair of input grids and an outputelectrode, "means applying angular velocity-modulated signals of a predetermined frequency deviation to an input grid of each tube, a separate resonant circuit connected to each output electrode of each tube, said resonant circuits being tuned :to respectively higher and 'lower frequencies than the mean frequency of said applied signals,. coupling connections from each resonant circuit to the second input electrode of each oppositetube, means for deriving from the output resonant circuit of one of said tubes signals whose frequency'deviation is materially reduced' relative to' said predetermined frequency deviation, and means for adjusting the relative selectivities of said two resonant circuits 18. Are-entrant circuit comprising a, pair of electron dischargetubes each having a pair of input electrodes and an output electrode, means applying signals of a given frequency variation to an'inputelectrode of each tube, a separate output circuitconne'c'ted to each output electrode of each tube, said output circuits being tuned to respectively different frequencies than the mean frequency of said applied signals, a coupling connection from each output circuit to thesecond input electrode of the opposite tube, means for deriving from at least one of theoutput circuits signals whose frequency variation has been reduced relative to said given variation, and said output circuits being of different selectivities, said selectivities being related in a predetermined manner to determine the degree of said frequency variation reduction.

' MURRAY G. CROSBY. 

